docs: Update spec links to point to Tendermint repo

[thanethomson]

Follows from #1204.

We currently do not make the Tendermint spec available via the docs site (docs.tendermint.com), and the spec from master is no longer relevant. We also keep the specification relevant to the specific Tendermint release series on each release branch, so it's easier to see diffs in the specs between major releases of Tendermint.

e.g.

--- https://docs.tendermint.com/master/spec/abci/abci.html#beginblock
+++ https://github.com/tendermint/tendermint/blob/v0.34.x/spec/abci/abci.md#beginblock